1. Field of Invention
This invention relates to a nuclear reactor refueling machine load control system. Such systems monitor the load imposed on the refueling machine as it inserts and removes fuel assemblies from a reactor core immersed in a shielding pool of water and terminates operation of the refueling machine when the sensed load exceeds specific limits indicative of a snag or obstruction in the path of the fuel assembly. More particularly, the invention relates to such a system which more accurately determines the load by taking into account the variations in buoyancy resulting from changes in the degree of immersion of the mast lifting the fuel assemblies, and the change in tension load imposed by spring biased reels for hoses and cables providing pneumatic and electrical services to the grippers on the lower end of the mast. The invention is also directed to a novel technique of protecting the fuel assembly being transported by detecting deviations in the instantaneous load from a running average of the load.
2. Background Information
Fuel assemblies are inserted in and removed from the core of a nuclear reactor and moved within containment by a refueling machine. A typical refueling machine includes a movable mast suspended by a cable inside a fixed mast. The movable mast is equipped at its lower end with grippers which engage the upper end of a fuel assembly.
The fuel assemblies comprise a number of fuel rods connected together at intervals by grid straps. As the gripper mast is raised and lowered by the cable to remove and insert fuel assemblies in the reactor core, the grid straps can become hung-up on adjacent assemblies o other obstacles. Continued raising of a hung-up fuel assembly can impose damaging forces on the grid straps. Similarly, a continued attempt to lower a hung-up fuel assembly, applies more of the suspended load to the grid strap which is caught also resulting in damage to the fuel assembly.
In order to detect hung-up fuel assemblies, a load sensor monitors the suspended load and terminates raising of the mast when an overload limit is exceeded. Some systems also terminate lowering of the mast if an underload limit is exceeded. Such a system is disclosed in U.S. Pat. No. 4,487,741.
Currently, the overload and underload limits are fixed limits set by the manufacturer of the fuel cells. However, these fixed limits do not take into account the many variables encountered by a load control system. Foremost is the variation between the types of fuel assemblies. Typically, the fuel assemblies in a particular reactor all have the same basic configuration which includes thimble tubes interspersed in an array of fuel rods. In some assemblies, groups of control rods called rod control clusters are movably inserted in the thimble tubes for controlling the reactivity of the reactor. In other fuel assemblies, reactivity sources are inserted in the thimble tubes, in others burnable poison occupies the thimble tubes. In still others, the thimble tubes are merely plugged. These different inserts can cause variations of as much as about 160 pounds in a fuel assembly that nominally weighs about 1600 pounds dry. The fixed manufacturer limits do not take into account these variations in the weight of the different fuel assembly configurations.
Other variables which affect the sensed load on the refueling machine include buoyancy changes. While the fuel assemblies remain fully immersed in water which provides radiation shielding, the mast is only partially immersed when fully raised. This results in a decrease in the sensed load as the mast is lowered and more of its weight is supported by the buoyancy provided by the water. In addition, the grippers on the bottom of the movable mast are typically operated by compressed air through electrically controlled valves. The associated air hoses and electrical cables are wound on spring biased reels so that as the mast is lowered and the hose and electrical cable reel out, tension forces of increasing magnitude are applied to the mast. These tension forces also reduce the sensed load as the mast is lowered.
Neither the change in buoyancy nor the tension load applied by the hose and electrical cable reels is accounted for in present refueling machine load control systems.
On some refueling machines, playing out of the cable supporting the mast results in an increase in the sensed load as the mast is lowered. While this change in the sensed load is opposite to the buoyancy change and tension loading by the hose and electrical cable reels, the latter two effects dominate, so that even where the suspended cable load increases, the sensed load decreases as the mast is lowered.
Since the current refueling machine load control systems do not account for changes in buoyancy, tension loading or, where applicable, changes in hoist cable loading, they can provide overprotection under some circumstances and under protection in others. For instance, since buoyancy changes and tension loading reduce the sensed load for a nearly fully immersed mast, the system operates closer to the underload limit under these conditions. Thus, slight resistance to lowering the mast further can cause a nuisance trip. On the other hand, the reduction in the sensed load near the full down position requires increased resistance to raising the mast before the overload limit is exceeded. This can result in damage to the fuel assembly before movement of the mast is terminated.
There is a need, therefore, for a refueling machine load control system which more accurately detects hang-ups and obstructions in the path of fuel assemblies being transported by the machine.
There is also a need for such a system which takes into account the variations in configuration of the fuel assemblies.
There is a further need for such a system which accommodates for variations in load attributable to the changing degree of immersion of the suspended load in the shielding water covering the reactor core.
There is still further a need for such a system which takes into account the effects of the changing tension loading produced by the spring biased reels for the pneumatic hoses and electrical cables servicing the grippers which engage the fuel assemblies.
There is, in addition, a need for such a system which takes into account, where applicable, changes in the suspended load attributable to the cabling supporting the refueling machine mast.